1. Field of the Invention
The present invention relates to an image reading unit comprising an image reading element array having a plurality of linearly disposed photoelectric conversion elements, and an image reader for reading a reflection original, such as printed paper, and a transmission original, such as a photographic film, by using the image reading unit.
2. Description of the Related Art
Conventionally, when a transmission original, such as a photographic film, is read by an image reader of a flat head type having an original table glass, a method in which the transmission original placed in a transmission original guide unit is set on the original table glass and is illuminated from thereabove by a surface light source unit is mainly used. In general, on the assumption that the main originals to be read are reflection originals, the image reader of this type is designed so that the location where light is most properly focused is situated 0 mm from the top surface of the original table glass, that is, on the original table glass. In general, when transmission originals are read, they are set 0.5 mm above the original table glass surface by a transmission original guide in order to, for example, prevent the production of a Newton ring or to allow dirt to be picked up when they contact the glass surface.
When a lens of a reducing optical system using a charge-coupled device (CCD) is used, this method does not give rise to any practical problems even if the depth of field is large and the distance between the original table glass and a transmission original is on the order of 0.5 mm.
In recent years, in order to achieve size reduction, a reading unit of a 1× magnification optical system using a rod lens array has made its appearance. Since the depth of field of the rod lens array is small, when the distance between the original table glass and a transmission original is on the order of 0.5 mm, the reading unit has a problem in that an image goes out of focus.